The optical fine pattern lithography has now reached the limit to further development, and to keep the semiconductor-related industries still taking the part of traction in the economical and industrial field, development of a new paradigm to expand the horizon is a matter of great urgency. The nano-technology is supposed to break the deadlock, releasing the fine pattern lithography from the yoke of complicatedness, large-size and high cost. What is aimed at is to provide a three-dimensional nano-fabrication art to meet the demand for producing a variety of devices in small quantities (design change permitted and low cost). Particularly from the angle of “photo electronics” non-contamination and non-deficiency are absolutely required, and the “batch production of devices” and “controlling permitted on site” are required, also.
There are two requirements for the lithography; improvement of throughput (related to the degree of sensitivity in the resist) and improvement of resolution (resolving power of the resist). These factors need to be balanced. The electron beam is shorter in wavelength than the light, and therefore the resolution limit in the optical lithography can be overcome by using the electron beam. From the point of throughput's view the organic resist has been generally used in the electron beam lithography as is the case with the optical lithography. The inorganic resist is good in resolution, but it has not been used because of the low sensitivity. Among the organic resists PMMA is generally used; it is relatively low in sensitivity, but is good in resolution. What is aimed at in developing inorganic resists is to improve the sensitivity of inorganic resists to the extent that it is equal to or exceeds the sensitivity of PMMA.
Another problem in the electron beam lithography is the commonly called “proximity effect”, which is caused by the scattering of secondary electrons not only from the incident electron beam but also from the resist and the substrate. This makes the exposure region in the resist significantly larger than the size of the incident electron beam, and accordingly the resolution of inter-line space is lowered. In the hope of reducing the proximity effect every possible effort has been made. For one example, the proximity effect can be reduced by making an electron beam pass through a multi-layered resist prior to invasion in the substrate, thereby reducing the effective beam size thanks to the control of the refractive indexes of the electron beam. As a matter of fact, however, the proximity effect (enlargement of exposure area beyond the electron beam size) still limits the fine pattern lithography.
There are two kinds of resist sensitivity commonly taken into account: the digital type resist sensitivity and the analog type resist sensitivity. The digital type resist sensitivity shows a sudden change at a certain critical value, depending on the dose of the electron beam energy whereas the analog type resist sensitivity shows continuous change with the dose of the electron beam energy within a certain limited range. The digital type resist is advantageous to the sub-micron fine pattern lithography because of easiness in attaining a required space resolution. The “hard” reacted region formed therein is used as a mask, which functions to selectively permit etching or growth (called “regrowth”) at a subsequent step. On the other hand, although the analog type resist is limited in space resolution, it can work as a “soft” mask in the subsequent processing, and therefore, it is used in fine pattern lithography while controlling the difference of elevation. To provide three-dimensional very fine structures as desired it is necessary to improve the analog type resist in space resolution and behavior in subsequent proceedings.
The selective growth processing subsequent to the finishing of the mask pattern uses a growth method using a gas species whose surface diffusion length is long (CVD, GSMBE or CBE); the mask pattern is generally made by the optical lithography, and therefore, the mask width (the region width in which the growth is to be selectively suppressed) is very large, and to cause the selective growth in the non-masking region the atoms of the raw growth material projected on the mask need to be eliminated by diffusion. Such selective growth is applied to every kind of compound semiconductor including GaN, and is used in the Si process as one practice established for making three-dimensional structures. In respect of sub-micron and still smaller three-dimensional structural control there is a problem of the very fine mask region being buried by the increased surface diffusion length of the gas species. The surface diffusion length needs to be short (although still longer than the mask width) in case of a relatively small mask region.
Patent Application Laid-Open No. H8-172053 discloses a selective growth using CVD method, particularly the selective growth on a Group III-V compound semiconductor according to the metal-organic chemical vapor deposition method (hereinafter referred to as “MOCVD”).
The '053 publication uses the “MOCVD” method, and therefore the diffusion length of surface atoms is too long to permit the selective growth on a Group III-V compound semiconductor substrate, and therefore a high-density integration as desired cannot be attained thereon. The grown film thickness cannot be equal in the nano-order in all crystal growing directions, either.
In view of the above, one object of the present invention is to provide a three-dimensional very fine pattern lithography facilitating the “on-site”, high-density integration on a substrate while its circuit patterns are controlled to be constant in crystal thickness in the nano-order in the crystal growth directions. Another object of the present invention is to provide a very fine, high-density three-dimensional structure.